Overview
Code division multiple access (CDMA) is a modulation and multiple-access scheme based on spread-spectrum communication. In this scheme, multiple users share the same frequency band at the same time, by spreading the spectrum of their transmitted signals, so that each user's signal is pseudo-orthogonal to the signals of the other users.

CDMA Signals
In a CDMA system, each signal consists of a different pseudorandom binary sequence (called the spreading code) that modulates a carrier, spreading the spectrum of the waveform. A large number of CDMA signals share the same frequency spectrum. If CDMA is viewed in either the frequency or time domain, the multiple access signals overlap with each other. However, the use of statistically orthogonal spreading codes separates the various signals in the code space.

CDMA Receivers
A CDMA receiver separates the signals by means of a correlator that uses the particular binary sequence to despread the signal and collect the energy of the desired signal. Other users' signals, whose spreading codes do not match this sequence, are not despread in bandwidth and, as a result, contribute only to the noise. These signals represent a self-interference generated by the system. The output of the correlator is sent to a narrow-bandwidth filter. The filter allows all of the desired signal's energy to pass through, but reduces the interfering signal's energy by the ratio of the bandwidth before the correlator to the bandwidth after the correlator. This reduction greatly improves the signal-to-interference ratio of the desired signal. This ratio is also known as the processing gain. The signal-to-noise ratio is determined by the ratio of the desired signal power to the sum of all of the other signal powers. It is enhanced by the processing gain or the ratio of spread bandwidth to baseband data rate.

CDMA Channel Assignments
A CDMA digital cellular waveform design uses a pseudorandom noise (PN) sequence to spread the spectrum. The sample rate of the spreading sequence (called the chip rate) is chosen so that the bandwidth of the filtered signal is several times the bandwidth of the original signal.
A typical system might use multiple PN sequences. In addition, it might use repeated spreading codes of known lengths to ensure orthogonality between signals intended for different users. The channel assignment is essentially determined by the set of codes that are used for that particular link. Thus, the signal transmitted at any time in a logical channel is determined by:

* The frequency of operation for the base station
* The current symbol
* The specific orthogonal spreading code assigned for the logical channel
* The PN spreading code

presented by:
MALLAPI SHYAM REDDY

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Code Division Multiple Access (CDMA)
Overview
Code division multiple access (CDMA) is a modulation and multiple-access scheme based on spread-spectrum communication. In this scheme, multiple users share the same frequency band at the same time, by spreading the spectrum of their transmitted signals, so that each user's signal is pseudo-orthogonal to the signals of the other users.
CDMA Signals
In a CDMA system, each signal consists of a different pseudorandom binary sequence (called the spreading code) that modulates a carrier, spreading the spectrum of the waveform. A large number of CDMA signals share the same frequency spectrum. If CDMA is viewed in either the frequency or time domain, the multiple access signals overlap with each other. However, the use of statistically orthogonal spreading codes separates the various signals in the code space.
CDMA Receivers
A CDMA receiver separates the signals by means of a correlator that uses the particular binary sequence to despread the signal and collect the energy of the desired signal. Other users' signals, whose spreading codes do not match this sequence, are not despread in bandwidth and, as a result, contribute only to the noise. These signals represent a self-interference generated by the system. The output of the correlator is sent to a narrow-bandwidth filter. The filter allows all of the desired signal's energy to pass through, but reduces the interfering signal's energy by the ratio of the bandwidth before the correlator to the bandwidth after the correlator. This reduction greatly improves the signal-to-interference ratio of the desired signal. This ratio is also known as the processing gain. The signal-to-noise ratio is determined by the ratio of the desired signal power to the sum of all of the other signal powers. It is enhanced by the processing gain or the ratio of spread bandwidth to baseband data rate.
CDMA Channel Assignments
A CDMA digital cellular waveform design uses a pseudorandom noise (PN) sequence to spread the spectrum. The sample rate of the spreading sequence (called the chip rate) is chosen so that the bandwidth of the filtered signal is several times the bandwidth of the original signal.
A typical system might use multiple PN sequences. In addition, it might use repeated spreading codes of known lengths to ensure orthogonality between signals intended for different users. The channel assignment is essentially determined by the set of codes that are used for that particular link. Thus, the signal transmitted at any time in a logical channel is determined by:
* The frequency of operation for the base station
* The current symbol
* The specific orthogonal spreading code assigned for the logical channel
* The PN spreading code

Code Division Multiple Access (CDMA)

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What is CDMA


CDMA used extensively in radio frequency communication systems, especially in 2G and 3G cellular telephone networks.

Basic Advantage is the way it handles a finite BW among a large number of users (more users can transmit the same data over the same Bandwidth)


Architecture


.Two frequency channels
One control channel
One data channel
.Spreading code
A common spreading code is used by all nodes over the control channel
Several terminal-specific codes can be used over the data channel
.Signal over the control channel is completely orthogonal to any signal over the data channel

How does CDMA work

The principle is the same as in wireless application. Each user is assigned a unique code (spreading length -L-) which is multiplied by each bit. This code is only known to the receiver in order to demodulate the data.

The most important part for correct detection is the code. This code must be uncorrelated from other user’s codes and be orthogonal.

Advantage Of CDMA

Many users of CDMA use the same frequency, TDD or FDD may be used

Multipath fading may be substantially reduced because of large signal bandwidth

No absolute limit on the number of users

Easy addition of more users

Impossible for hackers to decipher the code sent

Better signal quality